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Intro to FSO

DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN ALTERNATIVE MODULATION SCHEME FOR FREE SPACE OPTICS (FSO). Fiber Optic Cable. Intro to FSO. Intra-city Fiber Optic Links. The Reasoning. High-speed Access The Last Mile Problem?

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Intro to FSO

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  1. DIGITAL PULSE INTERVAL MODULATION (DPIM) AS AN ALTERNATIVE MODULATION SCHEME FOR FREE SPACE OPTICS (FSO)

  2. Fiber Optic Cable Intro to FSO • Intra-city Fiber Optic Links

  3. The Reasoning • High-speed Access • The Last Mile Problem? Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam propagation @ 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego

  4. The Solution • Free Space Optics Picture taken from: I. I. Kim, and E. Korevaar, Availability of Free Space Optics (FSO) and hybrid FSO/RF systems, Optical Access Incorporated, San Diego

  5. The Solution (cont’d) • High-speed Access (cont’d) Picture taken from: I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam propagation @ 785nm and 1550nm in fog and haze for optical wireless communications, Optical Access Incorporated, San Diego

  6. The Solution (cont’d) • Typical FSO Laser/Photodiode Systems Photos taken from: http://www.systemsupportsolutions.com

  7. FSO Limitations • Power Link Budget Equation • PTX – Power Transmitted • PRX – Power Received • dTX – Transmit Aperture Diameter (m) • dRX – Receive Aperture Diameter (m) • D – Beam Divergence (mrad) • R – Range (km) • – atmospheric attenuation factor (dB/km)

  8. FSO Limitations (cont’d) • Atmospheric Attenuation Table taken from: I. I. Kim, and E. Korevaar, Availability of Free Space Optics (FSO) and hybrid FSO/RF systems, Optical Access Incorporated, San Diego

  9. Picture taken from: TD. A. Rockwell, and G. S. Mecherle, Optical Wireless: Low-cost, Broadband, Optical Access, Fsona Communication Corporation, Richmond, BC FSO Limitations (cont’d) • TX/RX Alignment • TX/RX Misalignment

  10. RF Back-up (Hybrid FSO/RF) Limitation Solutions • Active Beam Tracking

  11. Limitation Solutions (cont’d) • Increase Laser Power • Higher power received • Higher power per unit area • Operating @ 1550nm instead of 800nm • Increase Average Power Efficiency (APE) • Pulse Modulation Schemes can provide higher average power efficiency at the expense of higher BW requirement • Hence, increase Peak-APE

  12. Limitation Solutions (cont’d) • On-Off Keying (OOK) • Simplest solution based on intensity modulation • ‘0’ – zero intensity, ‘1’ positive intensity • Popular Pulse Time Modulation Schemes for OC • Pulse Position Modulation (PPM) • Pulse Interval Modulation (PIM)

  13. Pulse Time Modulation • PPM • Higher average power efficiency than OOK • Increases system complexity due to symbol-level synchronization. • DPIM • Higher APE than OOK but a bit lower than PPM • No symbol-level synchronization required • Higher Information capacity • Data encoded as a number of time intervals between successive pulses • Simplified receiver structure

  14. Table taken from: A.R. Hayes, Z. Ghassemlooy, and N.L. See, The Effect of Baseline Wander on the Performance of Digital Pulse Interval Modulation, 1999 IEEE Pulse Time Modulation (cont’d)

  15. Pulse Time Modulation (cont’d) • M = log2L Picture Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland

  16. Pulse Time Modulation (cont’d) • Bandwidth and Power Efficiency Comparisons Table Taken form: J. Zhang, Modulation Analysis for Outdoors Applications of Optical Wireless Communications, Nokia Networks Oy, Finland

  17. Conclusion • Power Increased by DPIM @ the cost of increased BW. • Higher power means more power received @ the receiver @ high levels of attenuation and misalignment between TX/RX • Major FSO benefit: reliable link connection and/or increased distance between TX/RX for certain cities

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